MAPK pathways activate and phosphorylate the osteoblast-specific transcription factor, Cbfa1

Sphingomyelinase amplifies BMP-4-induced osteocalcin synthesis in osteoblasts: role of ceramide

We previously reported that extracellular sphingomyelinase induces sphingomyelin hydrolysis in osteoblast-like MC3T3-E1 cells and that mitogen-activated protein (MAP) kinases are involved in bone morphogenetic protein (BMP)-4-stimulated osteocalcin synthesis in these cells. In the present study, we investigated whether sphingomyelinase affects BMP-4-stimulated synthesis of osteocalcin in osteoblast-like MC3T3-E1 cells. Sphingomyelinase significantly enhanced the BMP-4-stimulated osteocalcin synthesis. Among sphingomyelin metabolites, C(2)-ceramide enhanced the BMP-4-stimulated osteocalcin synthesis while sphingosine and sphingosine 1-phosphate had little effect on the synthesis. D-erythro-MAPP, an inhibitor of ceramidase, amplified the sphingomyelinase-effect on the osteocalcin synthesis. C(2)-ceramide suppressed the BMP-4-induced phosphorylation of p44/p42 MAP kinase, while having little effect on the phosphorylation of Smad1 and p38 MAP kinase. Taken together, our results strongly suggest that extracellular sphingomyelinase enhances the BMP-stimulated osteocalcin synthesis via ceramide in osteoblasts and that the effect of ceramide is exerted at a point upstream from p44/p42 MAP kinase.

Molecular basis of co-targeting prostate tumor and stroma

Prostate cancer is one of the leading causes of cancer death in Northern American men. The lethal phenotypes of human prostate cancer are characterized by progression to androgen-independence (Al) and a propensity to form osseous metastases. In approximately 80% of cases, prostate cancer colonizes bone and elicits a characteristic osteoblastic reaction. The bone metastases are initially sensitive to androgen deprivation treatments, but with time the cancer will eventually progress into an Al stage for which there is currently no effective treatment. Once initial hormonal therapy has failed, median survival of prostate cancer patients with bone metastases is less than 1 year (Tu et al. [2001] Lancet 357:336-341). Novel therapeutic and preventive strategies are needed to decrease morbidity and mortality of this disease.

Both the Smad and p38 MAPK pathways play a crucial role in Runx2 expression following induction by transforming growth factor-beta and bone morphogenetic protein

The Runx family of transcription factors plays pivotal roles during normal development and in neoplasias. In mammals, Runx family genes are composed of Runx1 (Pebp2alphaB/Cbfa2/Aml1), Runx2 (Pebp2alphaA/Cbfa1/Aml3) and Runx3 (Pebp2alphaC/Cbfa3/Aml2). Runx1 and Runx3 are known to be involved in leukemogenesis and gastric carcinogenesis, respectively. Runx2, on the other hand, is a common target of transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) and plays an essential role in osteoblast differentiation. Runx2 is induced by the receptor-activated Smad; Runx2 mediates the blockage of myogenic differentiation and induces osteoblast differentiation in C2C12 pluripotent mesenchymal precursor cells. However, Smad does not directly induce Runx2 expression; an additional step of de novo protein synthesis is required. Here we report that Smad-induced junB functions as an upstream activator of Runx2 expression. Furthermore, not only the Smad pathway but also the mitogen-activated protein kinase (MAPK) cascades are involved in the induction of Runx2 by TGF-beta1 and BMP-2. Our results demonstrate that following TGF-beta and BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation.

Continuous inhibition of MAPK signaling promotes the early osteoblastic differentiation and mineralization of the extracellular matrix

We screened the small molecule compounds that stimulate osteogenesis by themselves or promote bone morphogenetic protein (BMP)-induced bone formation. We found that a specific inhibitor for MAPK/extracellular signal-regulated kinase kinase (MEK)-1, promoted the early osteoblastic differentiation and mineralization of extracellular matrix (ECM) in C2Cl2 pluripotent mesenchymal cells treated with recombinant human BMP-2 (rhBMP-2) and MC3T3-E1 preosteoblastic cells. ALP activity was synergistically increased by the treatment with a specific MEK-1 inhibitor PD98059 and rhBMP-2 in both cell lines. Twenty-five micromolar PD98059 promoted mineralization of ECM in rhBMP-2-treated C2Cl2 cells and MC3T3-E1 cells. In contrast, PD98059 reduced osteocalcin (OCN) secretion and its transcriptional level in rhBMP-2-treated C2Cl2 cells but increased its secretion and mRNA level in MC3T3-E1 cells. Stable expression of a dominant-negative MEK-1 mutant in C2Cl2 cells represented high ALP activity and low osteocalcin production in the presence of rhBMP-2, while a constitutively active mutant of MEK-1 attenuated both of them. Together, our results indicated that BMP-2-induced mineralization of ECM of pluripotent mesenchymal stem cells and preosteoblastic cells could be controlled by a fine tuning of the MAPK signaling pathway. Further, MEK-1 inhibitors would be useful for the promotion of bone formation, for instance, the treatments for delayed fracture healing or advance of localized osteoporotic change after fracture healing.

Fibroblast growth factor 2 induction of the osteocalcin gene requires MAPK activity and phosphorylation of the osteoblast transcription factor, Cbfa1/Runx2

Fibroblast growth factor 2 (FGF-2) is an important regulator of bone formation and osteoblast activity. However, its mechanism of action on bone cells is largely unknown. A major route for FGF signaling is through the mitogen-activated protein kinase (MAPK) pathway. We showed recently that this pathway is important for activation and phosphorylation of Cbfa1/Runx2, an osteoblast-related transcription factor (Xiao, G., Jiang, D., Thomas, P., Benson, M. D., Guan, K., Karsenty, G., and Franceschi, R. T. (2000) J. Biol. Chem. 275, 4453-4459). The present study examined the mechanism of FGF-2 regulation of the mouse osteocalcin gene in MC3T3-E1 preosteoblastic cells. FGF-2 stimulated osteocalcin mRNA and promoter activity in a dose- and time-dependent manner in MC3T3-E1 preosteoblastic cells. Similar results were obtained in mouse bone marrow stromal cells. This stimulation required Runx2 and its DNA binding site in the osteocalcin promoter. FGF-2 also dramatically increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) followed by phosphorylation of Runx2. Furthermore, a specific ERK1/2 phosphorylation inhibitor, U0126, completely blocked both FGF-2-stimulated Runx2 phosphorylation and osteocalcin promoter activity, indicating that this regulation requires the MAPK pathway. Deletion studies showed that the C-terminal PST domain of Runx2 is required for the FGF-2 response. This study is the first demonstration that Runx2 is phosphorylated and activated by FGF-2 via the MAPK pathway and suggests that FGF-2 plays an important role in regulation of Runx2 function and bone formation.

Bone morphogenetic protein 2 induces dental follicle cells to differentiate toward a cementoblast/osteoblast phenotype

When triggered appropriately, dental follicle cells are considered to be able to differentiate toward a cementoblast/osteoblast phenotype. However, factors and mechanisms regulating follicle cell differentiation remain undefined. This study focused on determining the ability of bone morphogenetic protein (BMP) 2 to promote the differentiation of follicle cells and periodontal ligament (PDL) cells along a cementoblast/ osteoblast pathway. Follicle cells and PDL cells were isolated from the first molar region of CD-1 mice and immortalized with SV40. Both cell types expressed BMP-4 and BMP receptors (BMPR) IA and II, but only follicle cells expressed BMP-2 mRNA. Cells were exposed to recombinant human BMP (rhBMP)-2 (0-100 ng/ml) and Northern blots were used to determine the expression of mineral-associated markers. BMP-2, in a dose- and time-dependent manner, induced cementoblast/osteoblast differentiation of follicle cells, as reflected by enhanced core binding factor alpha (Cbfal), bone sialoprotein (BSP), and osteocalcin (OCN) mRNA expression and enhanced mineral formation. U0126, a specific inhibitor of MEK-1/2 members of the MAPK family, abolished BMP-2-mediated expression of BSP and OCN. In contrast, exposure of PDL cells to BMP-2 resulted in modest expression of OCN and minimal promotion of mineralization. These results suggest that BMP-2 triggers follicle cells to differentiate toward a cementoblast/osteoblast phenotype and that the MAPK pathway is involved.

Bone morphogenetic protein 2 induces cyclo-oxygenase 2 in osteoblasts via a Cbfal binding site: role in effects of bone morphogenetic protein 2 in vitro and in vivo

We tested the hypothesis that induction of cyclo-oxygenase (COX) 2 mediates some effects of bone morphogenetic protein (BMP) 2 on bone. BMP-2 induced COX-2 mRNA and prostaglandin (PG) production in cultured osteoblasts. BMP-2 increased luciferase activity in calvarial osteoblasts from mice transgenic for a COX-2 promoter-luciferase reporter construct (Pluc) and in MC3T3-E1 cells transfected with Pluc. Deletion analysis identified the -300/-213-bp region of the COX-2 promoter as necessary for BMP-2 stimulation of luciferase activity. Mutation of core-binding factor activity 1 (muCbfal) consensus sequence (5'-AACCACA3') at -267/-261 bp decreased BMP-2 stimulation of luciferase activity by 82%. Binding of nuclear proteins to an oligonucleotide spanning the Cbfal site was inhibited or supershifted by specific antibodies to Cbfal. In cultured osteoblasts from calvariae of COX-2 knockout (-/-) and wild-type (+/+) mice, the absence of COX-2 expression reduced the BMP-2 stimulation of both ALP activity and osteocalcin mRNA expression. In cultured marrow cells flushed from long bones, BMP-2 induced osteoclast formation in cells from COX-2(+/+) mice but not in cells from COX-2(-/-) mice. In vivo, BMP-2 (10 microg/pellet) induced mineralization in pellets of lyophilized collagen implanted in the flanks of mice. Mineralization of pellets, measured by microcomputed tomography (microCT), was decreased by 78% in COX-2(-/-) mice compared with COX-2(+/+) mice. We conclude that BMP-2 transcriptionally induces COX-2 in osteoblasts via a Cbfal binding site and that the BMP-2 induction of COX-2 can contribute to effects of BMP-2 on osteoblastic differentiation and osteoclast formation in vitro and to the BMP-2 stimulation of ectopic bone formation in vivo.

Regulation of collagenase-3 and osteocalcin gene expression by collagen and osteopontin in differentiating MC3T3-E1 cells

Both collagenase-3 and osteocalcin mRNAs are expressed maximally during the later stages of osteoblast differentiation. Here, we demonstrate that collagenase-3 mRNA expression in differentiating MC3T3-E1 cells is dependent upon the presence of ascorbic acid, is inhibited in the presence of the collagen synthesis inhibitor, 3,4-dehydroproline, and is stimulated by growth on collagen in the absence of ascorbic acid. Transient transfection studies show that collagenase-3 promoter activity increases during cell differentiation and requires the presence of ascorbic acid. Additionally, we show that, in differentiating MC3T3-E1 cells, collagenase-3 gene expression increases in the presence of an anti-osteopontin monoclonal antibody that binds near the RGD motif of this protein, whereas osteocalcin expression is inhibited. Furthermore, an RGD peptidomimetic compound, designed to block interaction of ligands to the alpha(v) integrin subunit, increases osteocalcin expression and inhibits collagenase-3 expression, suggesting that the RGD peptidomimetic initiates certain alpha(v) integrin signaling in osteoblastic cells. Overall, these studies demonstrate that stimulation of collagenase-3 expression during osteoblast differentiation requires synthesis of a collagenous matrix and that osteopontin and alpha(v) integrins exert divergent regulation of collagenase-3 and osteocalcin expression during osteoblast differentiation.

Bone morphogenetic protein-transduced human fibroblasts convert to osteoblasts and form bone in vivo

Experimental cell or ex vivo gene therapy for localized bone formation typically uses osteoprogenitor cells propagated from periosteum or bone marrow. Both require bone or marrow biopsies to obtain cells. We have demonstrated that implantation of gingival or dermal fibroblasts transduced with BMP ex vivo, using a recombinant adenovirus (AdCMVBMP) attached to porous biodegradable scaffolds, form bone in vivo. Here we show that BMP-7-transduced fibroblasts suspended in injectable thermoset hydrogels form complete ossicles on subcutaneous injection and repair segmental defects in rat femurs. Bone formation was preceded by an intermediate cartilage stage. To determine the fate of the implanted transduced cells, thermoset hydrogel suspensions of ex vivo BMP-7-transduced or nontransduced fibroblasts were placed in diffusion chambers and implanted to allow development in vivo without direct contact with host cells. Only the BMP-transduced fibroblasts formed bone within the diffusion chambers in vivo, revealing that BMP transduction induces osteoblastic conversion of these cells.

Mechanical strain and fluid movement both activate extracellular regulated kinase (ERK) in osteoblast-like cells but via different signaling pathways

Extracellular regulated kinases (ERKs)-1 and -2 are members of the MAPK family of protein kinases involved in the proliferation, differentiation, and apoptosis of bone cells. We have shown previously that ROS 17/2.8 cells show increased activation of ERK-1 or -2, which is sustained for 24 h, when the strips onto which they are seeded are subjected to a 10 min period of cyclic four point bending that produces physiological levels of mechanical strain along with associated fluid movement of the medium. Movement of the strips through the medium without bending causes fluid movement without strain. This also increases ERK-1/2 activation, but in a biphasic manner over the same time period. Our present study investigates the role of components of signaling pathways in the activation of ERK-1/2 in ROS 17/2.8 cells in response to these stimuli. Using a range of inhibitors we show specific differences by which ERK-1 and ERK-2 are activated in response to fluid movement alone, compared with those induced in response to strain plus its associated fluid movement. ERK-1 activation induced by fluid movement was markedly reduced by nifedipine, and therefore appears to involve L-type calcium channels, but was unaffected by either L-NAME or indomethacin. This suggests independence from prostacyclin (PGI(2)) and nitric oxide (NO) production. In contrast, ERK-1 activation induced by application of strain (and its associated fluid disturbance) was abrogated by TMB-8 hydrochloride, L-NAME, and indomethacin. This suggests that strain-induced ERK-1 activation is dependent upon calcium mobilization from intracellular stores and production of NO and PGI(2). ERK-2 activation appears to be mediated by a separate mechanism in these cells. Its activation by fluid movement alone involved both PGI(2) and NO production, but its activation by strain was not affected by any of the inhibitors used. The G protein inhibitor, pertussis toxin, did not cause a reduction in the activation of ERK-1 or -2 in response to either stimulus. These results are consistent with earlier observations of ERK activation in bone cells in response to both strain (with fluid movement) and fluid movement alone, and further demonstrate that these phenomena stimulate distinct signaling pathways.

Glucocorticoid-induced osteoporosis in the rat is prevented by the tyrosine phosphatase inhibitor, sodium orthovanadate

Glucocorticoid-induced osteoporosis is characterized by decreased osteoblast numbers and a marked impairment of new bone formation. We found that, in vitro, dexamethasone inhibits both preosteoblast proliferation and mitogenic kinase activity in response to mitogens, and that inhibition of protein tyrosine phosphatases (PTPs) using sodium orthovanadate prevents this. Therefore, dexamethasone may act by either upregulating antiproliferative PTPs or downregulating promitogenic tyrosine-phosphorylated substrates. In this study, osteoporosis was induced in 3.5-month-old rats by subcutaneous injection with methylprednisolone 3.5 mg/kg per day for 9 weeks. Rats were treated with steroid alone or in combination with 0.5 mg/mL sodium orthovanadate, administered continuously in drinking water. Steroid-treated bones were significantly (p < 0.005) osteopenic (according to dual-energy X-ray absorptiometry) and physically weaker (p < 0.05) than controls. Quantitative bone histology confirmed a significant decrease in osteoid surfaces (p < 0.001), osteoblast numbers (p < 0.05), and rate of bone formation (p < 0.001). Concomitant treatment with vanadate largely prevented the densitometric, histologic, and physical abnormalities induced by prednisolone. This study supports our finding that PTPs are central to the negative regulation of osteoblast proliferation by glucocorticoids and, furthermore, suggests that PTP inhibitors such as sodium orthovanadate should be considered as novel anabolic agents for the treatment of steroid-induced osteoporosis.

The bone-specific transcriptional regulator Cbfa1 is a target of mechanical signals in osteoblastic cells

A primary goal of bone research is to understand the mechanism(s) by which mechanical forces dictate the cellular and metabolic activities of osteoblasts, the bone-forming cells. Several studies indicate that osteblastic cells respond to physical loading by transducing signals that alter gene expression patterns. Accumulated data have documented the fundamental role of the osteoblast-specific transcription factor Cbfa1 (core-binding factor) in osteoblast differentiation and function. Here, we demonstrate that low level mechanical deformation (stretching) of human osteoblastic cells directly up-regulates the expression and DNA binding activity of Cbfa1. This effect seems to be fine tuned by stretch-triggered induction of distinct mitogen-activated protein kinase cascades. Our novel finding that activated extracellular signal-regulated kinase mitogen-activated protein kinase physically interacts and phosphorylates endogenous Cbfa1 in vivo (ultimately potentiating this transcription factor) provides a molecular link between mechanostressing and stimulation of osteoblast differentiation. Elucidation of the specific modifiers and cofactors that operate in this mechanotranscription circuitry will contribute to a better understanding of mechanical load-induced bone formation which may set the basis for nonpharmacological intervention in bone loss pathologies.

The role of neurofibromin and melatonin in pathogenesis of pseudarthrosis after spinal fusion for neurofibromatous scoliosis

We might hypothesize that the high rate of pseudarthrosis after spinal fusion for neurofibromatous scoliosis is related to two factors: the absence of neurofibromin and melatonin deficiency. Loss of the up-regulation of neurofibromin during the healing process might abolish the bone-forming effects mediated through platelet-derived growth factor (PDGF) and transforming growth factor (TGF) beta1. The absence of neurofibromin might cause an increase in the Ras activity that increases the mitogen-activated protein kinase (MAPK) with resultant disturbance of the regulatory mechanism of core binding transcription factor (Cbfa 1) and increase of osteocalcin. These effects might inhibit bone formation. Melatonin deficiency might cause defective bone formation and favour excess fibrous tissue formation.

Age-related bone loss: old bone, new facts

The human skeleton serves several functions for the body: support, locomotion, protection of vital organs, and housing of bone marrow. Bone remodeling is the result of the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the end result being the maintenance of the bone architecture and to maintain systemic calcium homeostasis. In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation is affected by the reduction of osteoblast differentiation, activity, and life span which is further potentiated in the perimenopausal years with hormone deprivation and increased osteoclast activity. Age-related bone loss is thus not only a consequence of hormone deprivation, but also the result of changes in bone formation and cell-cell interactions with a unique pathophysiology. In this review, we describe the cellular and metabolic changes associated with aging bone and present recent evidence regarding cell differentiation within the bone marrow. We also consider the mechanism of programmed cell death, apoptosis, as being an important determinant of aging in bone as well as describe possible future interventions to prolong the life span of osteoblasts.

Engineering gene expression and protein synthesis by modulation of nuclear shape

The current understanding of the relationships between cell shape, intracellular forces and signaling, nuclear shape and organization, and gene expression is in its infancy. Here we introduce a method for investigating gene-specific responses in individual cells with controlled nuclear shape and projected area. The shape of the nuclei of primary osteogenic cells were controlled on microfabricated substrata with regiospecific chemistry by confining attachment and spreading of isolated cells on adhesive islands. Gene expression and protein synthesis were altered by changing nuclear shape. Collagen I synthesis correlated directly with cell shape and nuclear shape index (NSI), where intermediate values of nuclear distension (6 < NSI < 8) promoted maximum synthesis. Osteocalcin mRNA, a bone-specific differentiation marker, was observed intracellularly by using reverse transcription in situ PCR at 4 days in cells constrained by the pattern and not detected in unconstrained cells of similar projected area, but different NSI. Our data supports the concept of gene expression and protein synthesis based on optimal distortion of the nucleus, possibly altering transcription factor affinity for DNA, transport to the nucleus, or nuclear matrix organization. The combination of microfabricated surfaces, reverse transcription in situ PCR, and NSI measurement is an excellent system to study how transcription factors, the nuclear matrix, and the cytoskeleton interact to control gene expression and may be useful for studying a wide variety of other cell shape/gene expression relationships.

Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. PTH and PTH-related protein (PTHrP) indirectly activate osteoclasts resulting in increased bone resorption. During this process, PTH changes the phenotype of the osteoblast from a cell involved in bone formation to one directing bone resorption. In addition to these catabolic effects, PTH has been demonstrated to be an anabolic factor in skeletal tissue and in vitro. As a result, PTH has potential medical application to the treatment of osteoporosis, since intermittent administration of PTH stimulates bone formation. Activation of osteoblasts by PTH results in expression of genes important for the degradation of the extracellular matrix, production of growth factors, and stimulation and recruitment of osteoclasts. The ability of PTH to drive changes in gene expression is dependent upon activation of transcription factors such as the activator protein-1 family, RUNX2, and cAMP response element binding protein (CREB). Much of the regulation of these processes by PTH is protein kinase A (PKA)-dependent. However, while PKA is linked to many of the changes in gene expression directed by PTH, PKA activation has been shown to inhibit mitogen-activated protein kinase (MAPK) and proliferation of osteoblasts. It is now known that stimulation of MAPK and proliferation by PTH at low concentrations is protein kinase C (PKC)-dependent in both osteoblastic and kidney cells. Furthermore, PTH has been demonstrated to regulate components of the cell cycle. However, whether this regulation requires PKC and/or extracellular signal-regulated kinases or whether PTH is able to stimulate other components of the cell cycle is unknown. It is possible that stimulation of this signaling pathway by PTH mediates a unique pattern of gene expression resulting in proliferation in osteoblastic and kidney cells; however, specific examples of this are still unknown. This review will focus on what is known about PTH-mediated cell signaling, and discuss the established or putative PTH-regulated pattern of gene expression in osteoblastic cells following treatment with catabolic (high) or anabolic (low) concentrations of the hormone.

Inhibition of PI3K/p70 S6K and p38 MAPK cascades increases osteoblastic differentiation induced by BMP-2

Bone morphogenetic proteins (BMPs) transdifferentiate C2C12 cells from the myogenic to the osteogenic lineage. In this work we examine the role of the phosphatidylinositol 3-kinase/p70 S6 kinase (PI3K/p70 S6K) and p38 mitogen-activated protein kinase (p38 MAPK) cascades in the osteogenic effects of BMP-2. BMP-2 stimulated both cascades transiently (maximal at 1 h and decreasing thereafter). In contrast, BMP-2 had no effect on p42/p44 MAPK (Erks) stimulation. We also analyzed the effects of selective inhibitors of these pathways on the expression of osteogenic markers. Inhibitors of p38 MAPK (SB203580) or the PI3K/p70 S6K pathway (Ly294002 and rapamycin) not only fail to block the osteoblast phenotype induced by BMP-2, measured as induction of Cbfa1 expression and transcriptional activity, but also potentiate the effect of BMP-2 on late osteoblast markers, such as alkaline phosphatase activity and osteocalcin expression. These data suggest that, in contrast to their positive effect on myogenic differentiation, PI3K/p70 S6K and p38 MAPK cascades have a negative role in osteoblast differentiation.

Evidence for a role of p38 MAP kinase in expression of alkaline phosphatase during osteoblastic cell differentiation

In the present study, we investigate the implication of the mitogen-activated protein kinases (MAPKs) Erk, p38, and JNK in mediating the effect of fetal calf serum (FCS) on the differentiation of MC3T3-E1 osteoblast-like cells. Erk is stimulated by FCS in proliferating, early-differentiating, as well as in mature cells. Activation of p38 by FCS is not detected in proliferating cells but is observed as the cells differentiate. JNK is activated in response to FCS throughout the entire differentiation process, but a maximal stimulation is observed in early differentiating cells. The roles of Erk and p38 pathways in mediating MC3T3-E1 cell differentiation was determined using specific inhibitors such as U0126 and SB203580, respectively. These experiments confirmed that the Erk pathway is essential for mediating cell proliferation in response to FCS, but indicated that this MAP kinase has little effect in regulating the differentiation of MC3T3-E1 cells. In contrast, p38 only marginally influenced proliferation, but appeared to be critical for the control of alkaline phosphatase (ALP) expression in differentiating cells. Finally, results obtained with high doses of SB203580, which also affected JNK activity, suggest that p38 and/or JNK are probably also involved in the control of type 1 collagen and osteocalcin expression in differentiating cells. The data indicate that MAPKs regulate different stages of MC3T3-E1 cell development in response to FCS. Distinct MAPK pathways seem to independently modulate osteoblastic cell proliferation and differentiation, with Erk playing an essential role in cell replication, whereas p38 is involved in the regulation of ALP expression during osteoblastic cell differentiation. JNK is also probably involved in the regulation of osteoblastic cell differentiation, but its precise role requires further investigation.

Bone morphogenetic proteins, extracellular matrix, and mitogen-activated protein kinase signaling pathways are required for osteoblast-specific gene expression and differentiation in MC3T3-E1 cells

Osteoblasts secrete a complex extracellular matrix (ECM) containing collagenous and noncollagenous proteins, bone morphogenetic proteins (BMPs), and growth factors. Osteoblast-specific gene expression requires ascorbic acid (AA)-dependent assembly of a collagenous ECM. Matrix responsiveness requires an alpha2beta1 integrin-collagen interaction and mitogen-activated protein kinase (MAPK) activity, which phosphorylates and activates the osteoblast-specific transcription factor Cbfa1. This study examines interactions between this integrin/MAPK-mediated pathway and signals initiated by BMPs contained in the osteoblast matrix. MC3T3-E1 cells were shown to constitutively express BMP-2, BMP-4, and BMP-7. Noggin, a specific BMP inhibitor, reversibly blocked AA-induced gene expression, indicating that BMP production by MC3T3-E1 cells was necessary for differentiation. The ability of exogenously added BMP-2, BMP-4, or BMP-7 to stimulate osteocalcin (OCN) and bone sialoprotein (BSP) mRNAs or OCN promoter activity was synergistically increased in cells that were actively synthesizing an ECM (i.e., were grown in the presence of AA). A minimum of 4 days of ECM accumulation was required for this synergistic response to be observed. Neither BMP-7, AA, nor a combination of these two treatments had major effects on Cbfa1 messenger RNA (mRNA) or protein levels, as would be expected if regulation was mainly at the posttranscriptional level. U0126, a specific inhibitor of MAPK/extracellular signal-regulated kinase (MEK), blocked AA- or BMP-7/AA-dependent gene expression in a time- and dose-dependent manner that was closely correlated with inhibition of extracellular signal-regulated kinase (ERK) phosphorylation. This work establishes that autocrine BMP production as well as integrin-mediated cell-collagen interactions are both required for osteoblast differentiation, and both these pathways require MAP kinase activity.

Enhanced activity of osteoblast differentiation factor (PEBP2alphaA2/CBFa1) in affected sutural osteoblasts from patients with nonsyndromic craniosynostosis

OBJECTIVE

Nonsyndromic craniosynostosis is characterized by premature closure of one or more cranial sutures in infants. The purpose of this investigation was to evaluate cellular and molecular events that lead to pathogenesis of nonsyndromic craniosynostosis.

DESIGN

This study utilized discarded samples of normal and affected cranial sutures from 12 patients (7 boys, 5 girls) with nonsyndromic craniosynostosis.

RESULTS

Histological evaluation of affected sutures revealed complete osseous obliteration instead of a zone of connective tissue and osteogenic cells as seen in normal sutures. Although proliferation of normal and affected osteoblasts did not vary substantially, elevated osteocalcin production and increased in vitro bone nodule formation indicated that the differentiation and the bone-forming potential of affected osteoblasts was significantly higher than that of normal cells. We therefore investigated the levels and activity of Cbfa1, a transcription factor that plays an integral role in osteoblast differentiation. Northern blot analysis of messenger RNA from both normal and affected sutural osteoblasts revealed a twofold increase in the expression of Cbfa1 in affected cells. This increase in the level of Cbfa1 transcript correlated with an increase in its transcriptional activity on the osteocalcin gene promoter, as assessed using gene transfer methods.

CONCLUSION

Our results indicated that osteoblasts from synostosed sutures exhibit an increased potential for differentiation and bone formation. The increased level and activity of Cbfa1 could play a vital role in the aberrant function of these affected osteoblasts and may explain their altered behavior compared to the normal cells.

IL-1 induces collagenase-3 (MMP-13) promoter activity in stably transfected chondrocytic cells: requirement for Runx-2 and activation by p38 MAPK and JNK pathways

Osteoarthritic chondrocytes secrete matrix metalloproteinase-13 (MMP-13) in response to interleukin-1 (IL-1), causing digestion of type II collagen in cartilage. Using chondrocytic cells, we previously determined that IL-1 induced a strong MMP-13 transcriptional response that requires p38 MAPK, JNK and the transcription factor NF-kappaB. Now, we have studied the tissue-specific transcriptional regulation of MMP-13. Constitutive expression of the transcription factor Runx-2 correlated with the ability of a cell type to express MMP-13 and was required for IL-1 induction; moreover, Runx-2 enhanced IL-1 induction of MMP-13 transcription by synergizing with the p38 MAPK signaling pathway. Transiently transfected MMP-13 promoters were not IL-1 inducible. However, -405 bp of stably integrated promoter was sufficient for 5- to 6-fold IL-1 induction of reporter activity and this integrated reporter required the same p38 MAPK pathway as the endogenous gene. Finally, mutation of the proximal Runx binding site and the proximal AP-1 site blunted the transcriptional response to IL-1, and double mutation synergistically decreased reporter activity. In summary, our data suggest that the transcriptional MMP-13 response to IL-1 is controlled by the p38 pathway interacting at the MMP-13 promoter through the tissue-specific transcription factor Runx-2 and the ubiquitous AP-1 transcription factor.

1-(5-oxohexyl)-3,7-Dimethylxanthine, a phosphodiesterase inhibitor, activates MAPK cascades and promotes osteoblast differentiation by a mechanism independent of PKA activation (pentoxifylline promotes osteoblast differentiation)

We have investigated the effect of 1-(5-oxohexyl)-3,7-dimethylxanthine or pentoxifylline (PeTx), a nonselective phosphodiesterase inhibitor, on osteoblastic differentiation in vitro by using two mesenchymal cell lines, C3H10T1/2 and C2C12, which are able to acquire the osteoblastic phenotype in the presence of bone morphogenetic protein-2 (BMP-2). PeTx induced the osteoblastic markers, osteocalcin and Osf2/Cbfa1, in C3H10T1/2 and C2C12 cells and enhanced BMP-2-induced expression of osteocalcin, Osf2/Cbfa1, and alkaline phosphatase. This activity was partially attributed to the fact that PeTx is able to enhance BMP-2-induced Smad1 transcriptional activity. Although PeTx clearly stimulates PKA in these cells, neither pretreatment of cells with the PKA inhibitor H89 nor transfection with the specific PKA inhibitor PKI prevented the induction or enhancement of osteoblast markers by PeTx, demonstrating that these effects were independent of PKA activation. On the other hand, PeTx induced the activation of ERK1/2 and p38 kinase pathways independently of the activation of PKA. Selective inhibitors of these MAPK cascades prevented the induction of osteoblastic markers in cells treated with PeTx, suggesting that the activation of these two pathways plays a role in the effect of PeTx on osteoblastic differentiation.

Characterization of the upstream mouse Cbfa1/Runx2 promoter

Cbfa1 (or Runx2/AML-3/PEPB2alpha) is a transcriptional activator of osteoblastic differentiation. To investigate the regulation of Cbfa1 expression, we isolated and characterized a portion of the 5'-flanking region of the Cbfa1 gene containing its "bone-related" or P1 promoter and exon 1. We identified additional coding sequence in exon 1 and splice donor sites that potentially give rise to a novel Cbfa1 isoform containing an 18 amino acid insert. In addition, primer extension mapping identified in the Cbfa1 promoter a minor mRNA start site located approximately 0.8 kb 5' upstream of the ATG encoding the MASN/p57 isoform and approximately 0.4 kb upstream of the previously reported start site. A luciferase reporter construct containing 1.4 kb of the mouse Cbfa1 promoter was analyzed in Ros 17/2.8 and MC3T3-E1 osteoblast cell lines that express high levels of Cbfa1 transcripts. The activity of this construct was also examined in non-osteoblastic Cos-7 and NIH3T3 cells that do not express Cbfa1 and mesenchymal-derived cell lines, including CH3T101/2, C2C12, and L929 cells, that express low levels of mature Cbfa1 transcripts. The 1.4 kb 5' flanking sequence of the Cbfa1 gene directed high levels of transcriptional activity in Ros 17/2.8 and MC3T3-E1 osteoblasts compared to non-osteoblasts Cos-7 cells, but this construct also exhibited high levels of expression in C310T1/2, L929, and C2C12 cells as well as NIH3T3 cells. In addition, Cbfa1 mRNA expression, but not the activity of the Cbfa1 promoter, was upregulated in a dose-dependent manner in pluripotent mesenchymal C2C12 by bone morphogenetic protein-2 (BMP-2). These data indicate that Cbfa1 is expressed in osteogenic as well as non-osteogenic cells and that the regulation of Cbfa1 expression is complex, possibly involving both transcriptional and post-transcriptional mechanisms. Additional studies are needed to further characterize important regulatory elements and to identify additional regions of the promoter and/or post-transcriptional events responsible for the cell-type restricted regulation of Cbfa1 expression.

Focal adhesion kinase activity is required for bone morphogenetic protein--Smad1 signaling and osteoblastic differentiation in murine MC3T3-E1 cells

Cell-matrix interactions via integrins are essential for osteoblastic differentiation. We have shown that signals activated by aggregation of alpha2beta1-integrin with type I collagen are involved in the differentiation of osteoblastic MC3T3-E1 cells. Focal adhesion kinase (FAK) is an immediate downstream signal of the beta1-integrin, and inactivation of FAK has been shown to disrupt osteoblastic differentiation. To elucidate roles of FAK in osteoblastic cells, we examined MC3T3-E1 cells stably expressing antisense FAK (asFAK) messenger RNA (mRNA). Alkaline phosphatase (ALP) activity, an osteoblastic marker, did not increase in asFAK cells with a long-term culture until 21 days or in response to bone morphogenetic protein 2 (BMP-2). Treatment with BMP-2 also failed to stimulate the expression of osteocalcin in asFAK cells. In control MC3T3-E1 cells, BMP-2 induced translocation of Smad1 into nuclei to stimulate transcriptional activity of the Smad6 promoter gene that contains a Smad1 response element. In contrast, BMP-2 failed to increase transcriptional activity of Smad6 promoter gene in asFAK cells, although it induced nuclear translocation of Smad1. These results indicate that FAK was involved in Smad1-dependent transcriptional activity but not in nuclear translocation of Smad1 in osteoblastic cells. Hence, FAK activation by integrins might converge transcriptional activation by BMP of its target genes in osteoblastic cells. These observations suggest that FAK activity is essential for BMP-Smad signaling to stimulate osteoblastic differentiation.

Differential regulation of the two principal Runx2/Cbfa1 n-terminal isoforms in response to bone morphogenetic protein-2 during development of the osteoblast phenotype

Cbfa1/Runx2 is a transcription factor essential for bone formation and osteoblast differentiation. Two major N-terminal isoforms of Cbfa1, designated type I/p56 (PEBP2aA1, starting with the sequence MRIPV) and type II/p57 (til-1, starting with the sequence MASNS), each regulated by distinct promoters, are known. Here, we show that the type I transcript is constitutively expressed in nonosseous mesenchymal tissues and in osteoblast progenitor cells. Cbfa1 type I isoform expression does not change with the differentiation status of the cells. In contrast, the type II transcript is increased during differentiation of primary osteoblasts and is induced in osteoprogenitors and in premyoblast C2C12 cells in response to bone morphogenetic protein-2. The functional equivalence of the two isoforms in activation and repression of bone-specific genes indicates overlapping functional roles. The presence of the ubiquitous type I isoform in nonosseous cells and before bone morphogenetic protein-2 induced expression of the type II isoform suggests a regulatory role for Cbfa1 type I in early stages of mesenchymal cell development, whereas type II is necessary for osteogenesis and maintenance of the osteoblast phenotype. Our data indicate that Cbfa1 function is regulated by transcription, cellular protein levels, and DNA binding activity during osteoblast differentiation. Taken together, our studies suggest that developmental timing and cell type- specific expression of type I and type II Cbfa isoforms, and not necessarily molecular properties or sequences that reside in the N-terminus of Cbfa1, are the principal determinants of the osteogenic activity of Cbfa1.

Induction of osteoblast differentiation indexes by PTHrP in MG-63 cells involves multiple signaling pathways

Parathyroid hormone (PTH)-related peptide (PTHrP) can modulate the proliferation and differentiation of a number of cell types including osteoblasts. PTHrP can activate a G protein-coupled PTH/PTHrP receptor, which can interface with several second-messenger systems. In the current study, we have examined the signaling pathways involved in stimulated type I collagen and alkaline phosphatase expression in the human osteoblast-derived osteosarcoma cells, MG-63. By use of Northern blotting and histochemical analysis, maximum induction of these two markers of osteoblast differentiation occurred after 8 h of treatment with 100 nM PTHrP-(1-34). Chemical inhibitors of adenylate cyclase (H-89) or of protein kinase C (chelerythrine chloride) each diminished PTHrP-mediated type I collagen and alkaline phosphatase stimulation in a dose-dependent manner. These effects of PTHrP could also be blocked by inhibiting the Ras-mitogen-activated protein kinase (MAPK) pathway with a Ras farnesylation inhibitor, B1086, or with a MAPK inhibitor, PD-98059. Transient transfection of MG-63 cells with a mutant form of Galpha, which can sequester betagamma-subunits, showed significant downregulation of PTHrP-stimulated type I collagen expression, as did inhibition of phosphatidylinositol 3-kinase (PI 3-kinase) by wortmannin. Consequently, the betagamma-PI 3-kinase pathway may be involved in PTHrP stimulation of Ras. Collectively, these results demonstrate that, acting via its G protein-coupled receptor, PTHrP can induce indexes of osteoblast differentiation by utilizing multiple, perhaps parallel, signaling pathways.

AP-1 and Cbfa/runt physically interact and regulate parathyroid hormone-dependent MMP13 expression in osteoblasts through a new osteoblast-specific element 2/AP-1 composite element

The expression of MMP13 (collagenase-3), a member of the matrix metalloproteinase family, is increased in vivo as well as in cultured osteosarcoma cell lines by parathyroid hormone (PTH), a major regulator of calcium homeostasis. Binding sites for AP-1 and Cbfa/Runt transcription factors in close proximity have been identified as cis-acting elements in the murine and rat mmp13 promoter required for PTH-induced expression. The cooperative function of these factors in response to PTH in osteoblastic cells suggests a direct interaction between AP-1 and Cbfa/Runt transcription factors. Here, we demonstrate interaction between c-Jun and c-Fos with Cbfa/Runt proteins. This interaction depends on the leucine zipper of c-Jun or c-Fos and the Runt domain of Cbfa/Runt proteins, respectively. Moreover, c-Fos interacts with the C-terminal part of Cbfa1 and Cbfa2, sharing a conserved transcriptional repression domain. In addition to the distal osteoblast-specific element 2 (OSE2) element in the murine and rat mmp13 promoter, we identified a new proximal OSE2 site overlapping with the TRE motif. Both interaction of Cbfa/Runt proteins with AP-1 and the presence of a functional proximal OSE2 site are required for enhanced transcriptional activity of the mmp13 promoter in transient transfected fibroblasts and in PTH-treated osteosarcoma cells.

Activation of mitogen-activated protein kinase cascades is involved in regulation of bone morphogenetic protein-2-induced osteoblast differentiation in pluripotent C2C12 cells

Bone morphogenetic protein (BMP)-2, a member of the transforming growth factor-beta (TGF-beta) superfamily, is able to induce osteoblastic differentiation of C2C12 cells. Both Smad and mitogen-activated protein kinase (MAPK) pathways are essential components of the TGF-beta superfamily signaling machinery. Although Smads have been demonstrated to participate in the BMP-2-induced osteoblastic differentiation of C2C12 cells, the role of MAPK has not been addressed. This report shows that BMP-2 activates ERK and p38, but not JNK, in C2C12 cells. Pretreatment of cells with the p38 inhibitor, SB203580, dramatically reduced BMP-2-induced expression of the osteoblast markers alkaline phosphatase (ALP) and osteocalcin (OC). Nevertheless, overexpression of MKK3, a protein kinase that phosphorylates and activates p38, failed to induce ALP or OC expression in the absence of BMP-2, indicating that p38 activation is necessary but not sufficient for the acquisition of the osteoblast phenotype by these cells. Although ALP induction was increased slightly in the presence of PD-98059, a selective inhibitor of the ERK cascade, this compound significantly inhibited both steady-state and BMP-2-induced OC RNA levels. Our results indicate that p38 and ERK cascades play a crucial role in the osteoblast differentiation of C2C12 cells mediated by BMP-2.

Expression and regulation of Runx2/Cbfa1 and osteoblast phenotypic markers during the growth and differentiation of human osteoblasts

The runt family transcription factor (AML-3/PEBP2alphaA1/Cbfa1/RUNX2) plays a crucial role in formation of the mineralized skeleton during embryogenesis and regulates maturation of the osteoblast phenotype. Because steroid hormones and growth factors significantly influence growth and differentiation properties of osteoblasts, we addressed Cbfa1 as a target gene for regulation by dexamethasone (Dex), 1,25(OH)D(3) (vitamin D(3)), 17beta-estradiol, and transforming growth factor-beta1 (TGF-beta1). The representation of functional protein levels by Western blot analyses and gel mobility shift assays was examined during the growth and mineralization of several conditionally immortalized human osteoblast cell lines HOB 04-T8, 03-CE6, and 03-CE10, each representing different stages of maturation. In situ immunofluorescence demonstrates Cbfa1 is associated with nuclear matrix in punctate domains, some of which are transcriptionally active, colocalizing with phosphorylated RNA polymerase II. Although each of the cell lines exhibited different responses to the steroid hormones and to TGF-beta1, all cell lines showed a similar increase in Cbfa1 protein and DNA binding activity induced only by Dex. On the other hand, Cbfa1 mRNA levels were not altered by Dex treatment. This regulation of Cbfa1 by steroid hormones in human osteoblasts contrasts to modifications in Cbfa1 expression in primary rat calvarial osteoblasts and the mouse MC3T3-E1 osteoblast cell line. Thus, these results reveal multiple levels of regulation of Cbfa1 expression and activity in osteoblasts. Moreover, the data suggest that in committed human osteoblasts, constitutive expression of Cbfa1 may be required to sustain the osteoblast phenotype.

Neuropilin-1 expression in osteogenic cells: down-regulation during differentiation of osteoblasts into osteocytes

The expression of neuropilin-1 (NRP1), a recently described VEGF and semaphorin receptor expressed by endothelial cells (EC) but some non-EC types as well, was analyzed in osteoblasts in vitro and in vivo. Cultured MC3T3-E1 osteoblasts expressed NRP1 mRNA and bound VEGF(165) but not VEGF(121), characteristic of the VEGF isoform-specific binding of NRP1. These cells did not express VEGFR-1 or VEGFR-2 so that VEGF binding to osteoblasts was strictly NRP1-dependent. In a chick osteocyte differentiation system, NRP1 was expressed by osteoblasts but its expression was absent as the cells matured into osteocytes. Immunohistochemical localization of NRP1 within the developing bones of 36-day-old mice and embryonic Day 17 chicks demonstrated that NRP1 was expressed by osteoblasts migrating alongside invading blood vessels within the metaphysis of the growth plate, as well as by osteoblasts at the developing edge of trabeculae within the marrow cavity. On the other hand, NRP1 was not expressed by osteocytes in either species, consistent with the in vitro results. In addition to osteogenic cells, NRP1 expression by EC was observed throughout the bone. Together these results suggest that NRP1 might have a dual function in bone by mediating osteoblast function directly as well as angiogenesis.

Osteogenesis and bone-marrow-derived cells

This paper addresses some of the important aspects of stem cell commitment to the bone cell lineage examining the various types of precursor cells, their responses to cytokines and other extracellular influences, and recent observations on the biochemical and molecular control of lineage-specific gene expression. The process of osteopoiesis involves the proliferation and maturation of primitive precursor cells into functional osteoblasts. The bone cells purportedly originate from mesenchymal stem cells that commit to the osteogenic cell lineage becoming osteoprogenitor cells, preosteoblasts, osteoblasts, and osteocytes. Further understanding of this developmental process requires that lineage-specific markers be identified for the various populations of bone cells and their precursors, that cell separation techniques be established so that cells of the osteogenic lineage can be purified at different stages of differentiation, and that these isolated cells are studied under serum-free, chemically defined conditions.

Incadronate and etidronate accelerate phosphate-primed mineralization of MC4 cells via ERK1/2-Cbfa1 signaling pathway in a Ras-independent manner: further involvement of mevalonate-pathway blockade for incadronate

Two types of bisphosphonates (BPs), incadronate (INC) and etidronate (ETI) accelerated phosphate (Pi)-primed mineralization of MC4 cells in a subnanomolar dose range. Intracellular signaling pathways involved were examined. 1) The effect of INC but not ETI was partially suppressed by two mevalonate (MVA) pathway metabolites, farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP). 2) The BP-like accelerating effect was produced by statins and also by Toxin B, a Rho GTPases-specific inhibitor. 3) INC induced Cbfa1-nuclear localization within hours; and in an in vivo experiment using ovariectomized mice, its 3 weeks dosing exhibited the same effect in tibial extracts. 4) BPs promoted luciferase expression in murine p1.3-osteocalcin gene 2-luc and p6-osteoblast specific element 2-luc transfected cells, just as MVA, FPP and GGPP did independently and additively to INC. 5) BPs activated extracellular signal-regulated kinase (ERK1/2) in a Ras-independent manner within 5 min, and Pi was found to sensitize MC4 cells to BPs. MVA and its metabolites also activated ERKs but in a Ras-dependent manner and additively to INC. Ras dependency was determined using N17Ras-transfected cells. A MEK (MAP kinase-ERK kinase)-specific inhibitor PD98059 alone partly and with FPP completely blocked INC-induced mineralization. The results suggest that BPs act on Pi-sensitized MC4 cells to accelerate mineralization via nonRas-MEK-ERK1/2-Cbfa1 transactivation pathway and INC additionally acts by inhibiting the MVA pathway.

Role of N-cadherin and protein kinase C in osteoblast gene activation induced by the S252W fibroblast growth factor receptor 2 mutation in Apert craniosynostosis

Apert (Ap) syndrome is characterized by premature cranial suture ossification caused by fibroblast growth factor receptor 2 (FGFR-2) mutations. We studied the role of cadherins and signaling events in the phenotypic alterations induced by the Ap FGFR-2 S252W mutation in mutant immortalized fetal human calvaria osteoblasts. The FGFR-2 mutation caused increased expression of the osteoblast markers alkaline phosphatase (ALP), type 1 collagen (COLIA1), and osteocalcin (OC) in long-term culture. The mutation also increased cell-cell aggregation, which was suppressed by specific neutralizing anti-N- and anti-E-cadherin antibodies. Mutant osteoblasts showed increased N- and E-cadherin, but not N-cell adhesion molecule (N-CAM) messenger RNA (mRNA) and protein levels. This was confirmed in vivo by the abundant immunoreactive N- and E-cadherins in preosteoblasts in the Ap suture whereas N-CAM and alpha- and beta-catenins were unaffected. Neutralizing anti-N-cadherin antibody or N-cadherin antisense (AS) oligonucleotides but not anti-E-cadherin antibody or AS reduced ALP activity as well as ALP, COLIA1, and OC mRNA overexpression in mutant osteoblasts. Analysis of signal transduction revealed increased phospholipase Cgamma (PLCgamma) and protein kinase Calpha (PKCalpha) phosphorylation and increased PKC activity in mutant cells in basal conditions. Inhibition of PKC by calphostin C or the PKCalpha-specific inhibitor Gö6976 suppressed the increased N-cadherin mRNA and protein levels as well as the overexpression of ALP, COLIA1, and OC mRNA in mutant cells. Thus, N-cadherin plays a role in the activation of osteoblast differentiation marker genes in mutant osteoblasts and PKCalpha signaling appears to be involved in the increased N-cadherin and osteoblast gene expression induced by the S252W FGFR-2 mutation in human osteoblasts.

Erk is essential for growth, differentiation, integrin expression, and cell function in human osteoblastic cells

Extracellular signal-regulated kinases (Erks), members of the mitogen-activated protein kinase superfamily, play an important role in cell proliferation and differentiation. In this study we employed a dominant negative approach to determine the role of Erks in the regulation of human osteoblastic cell function. Human osteoblastic cells were transduced with a pseudotyped retrovirus encoding either a mutated Erk1 protein with a dominant negative action against both Erk1 and Erk2 (Erk1DN cells) or the LacZ protein (LacZ cells) as a control. Both basal and growth factor-stimulated MAPK activity and cell proliferation were inhibited in Erk1DN cells. Expression of Erk1DN protein suppressed both osteoblast differentiation and matrix mineralization by decreasing alkaline phosphatase activity and the deposition of bone matrix proteins. Cell adhesion to collagen, osteopontin, and vitronectin was decreased in Erk1DN cells as compared with LacZ cells. Cell spreading and migration on these matrices were also inhibited. In Erk1DN cells, expression of alphabeta(1), alpha(v)beta(3), and alpha(v)beta(5) integrins on the surface was decreased. Metabolic labeling indicated that the synthesis of these integrins was inhibited in Erk1DN cells. These data suggest that Erks are not only essential for the growth and differentiation of osteoblasts but also are important for osteoblast adhesion, spreading, migration, and integrin expression.

Gene expression of osteoclast differentiation factor is induced by lipopolysaccharide in mouse osteoblasts via Toll-like receptors

Osteoclast differentiation factor (ODF), a recently identified cytokine of the TNF family, is expressed as a membrane-associated protein in osteoblasts and stromal cells. ODF stimulates the differentiation of osteoclast precursors into osteoclasts in the presence of M-CSF. Here we investigated the effects of LPS on the gene expression of ODF in mouse osteoblasts and an osteoblast cell line and found that LPS increased the ODF mRNA level. A specific inhibitor of extracellular signal-regulated kinase or protein kinase C inhibited this up-regulation, indicating that extracellular signal-regulated kinase and protein kinase C activation was involved. A protein synthesis inhibitor, cycloheximide, rather enhanced the LPS-mediated increase of ODF mRNA, and both a neutralizing Ab of TNF-alpha and a specific inhibitor of PGE synthesis failed to block the ODF mRNA increase by native LPS. Thus, LPS directly induced ODF mRNA. Mouse osteoblasts and an osteoblast cell line constitutively expressed Toll-like receptor (TLR) 2 and 4, which are known as putative LPS receptors. ODF mRNA increases in response to synthetic lipid A were defective in primary osteoblasts from C3H/HeJ mice that contain a nonfunctional mutation in the TLR4 gene, suggesting that TLR4 plays an essential role in the process. Altogether, our results indicate that ODF gene expression is directly increased in osteoblasts by LPS treatment via TLR, and this pathway may play an important role in the pathogenesis of LPS-mediated bone disorders, such as periodontitis.

Interleukin-18 up-regulates osteoprotegerin expression in stromal/osteoblastic cells

Osteoprotegerin (OPG) and osteoclast differentiation factor (ODF) are crucial regulators of osteoclastogenesis. To determine the biological role of interleukin (IL)-18 produced by stromal/osteoblastic cells in osteoclastogenesis, we examined the effects of IL-18 on the OPG and ODF mRNA levels in these cells. When bone marrow stromal ST2 cells, osteoblastic MC3T3-E1 cells, and mouse calvarial osteoblasts were stimulated with IL-18, the expression of OPG mRNA, but not ODF mRNA, was transiently increased, its expression reaching a maximal level at 3 h after the beginning of the culture. In accordance with this observation, all these cells expressed the mRNAs of two IL-18 receptor components and MyD88, an adapter molecule involved in IL-18 signaling. Moreover, in these cells, mitogen-activated protein kinase was phosphorylated after stimulation with IL-18. These results suggest that stromal/osteoblastic cells are IL-18-responsive cells and that IL-18 may inhibit osteoclastogenesis by up-regulating OPG expression, without stimulation of ODF production, in stromal/osteoblastic cells.

Cbfa1: a molecular switch in osteoblast biology

During the past 4 years, our molecular understanding of osteoblast biology has made rapid progress due to the characterization of the function of one molecule, Cbfa1. This member of the runt/Cbfa family of transcription factors was first identified as the nuclear protein binding to an osteoblast-specific cis-acting element activating the expression of Osteocalcin, the most osteoblast-specific gene. Cbfa1 was then shown to regulate the expression of all the major genes expressed by osteoblasts. Consistent with this ability, genetic experiments identified Cbfa1 as a key regulator of osteoblast differentiation in vivo. Indeed, analysis of Cbfa1-deficient mice revealed that osteoblast differentiation is arrested in absence of Cbfa1, demonstrating both that it is required for this process and that no parallel pathway can overcome its absence. The importance of Cbfa1 in controlling osteoblast differentiation was further emphasized by the identification of Cbfa1 haploinsufficiency as the cause of cleidocranial dysplasia in humans and mice, a syndrome characterized by generalized bone defects. Lastly, Cbfa1 was shown to have a role beyond development and differentiation, regulating the rate of bone matrix deposition by differentiated osteoblasts. Thus, Cbfa1 is a critical gene not only for osteoblast differentiation but also for osteoblast function. These aspects, as well as the more recent progresses in understanding Cbfa1 biology, are the focuses of this review.

A fundamental transcription factor for bone and cartilage

Cbfa1-deficient mice were found to show a complete lack of bone formation owing to the maturational arrest of osteoblasts. Cbfa1 plays key roles in the determination of osteoblastic lineage from multipotential mesenchymal cells, their differentiation into mature osteoblasts, and transcriptional regulation of bone matrix-related genes. Cbfa1 positively regulates chondrocyte maturation and osteoclast differentiation and is required for vascular invasion into cartilage. Therefore, complete elucidation of the function of Cbfa1 and its signaling would be of great benefit in understanding skeletogenesis.